Rubber and similar polymers must be mixed with a selection of other ingredients to develop the properties necessary for specific applications. One of these ingredients is a vulcanizing agent. Elemental sulfur is by far the most widely used vulcanizing agent, especially in tires and other dynamic applications. While certain chemical compounds of sulfur can be used as sulfur donors to accomplish vulcanization, only elemental sulfur is believed to impart the optimum combination of properties for most of the tire. One of the most important properties required in tire rubber is fatigue resistance. The superior fatigue resistance achieved when using elemental sulfur instead of sulfur donors is reported in the "Natural Rubber Formulary" pp128-129, and again on pp180-181. Elemental sulfur is used in the rubber industry in two basic forms:
1. Ortho-rhombic (commonly called rhombic) crystals, consisting of molecules containing eight sulfur atoms per molecule in a ring-like structure. This form is referred to as normal, or soluble sulfur. Its main disadvantage is that it "blooms" in the unvulcanized rubber compound. PA0 2. Polymeric sulfur (or polysulfur, as it is sometimes called to distinguish it from organic polymers containing sulfur in their polymer chains) consists of molecules that contain long chains of sulfur atoms, usually thousands of sulfur atoms per molecule. At room and processing temperatures, these chains tend to revert to normal sulfur. This reversion can be deterred by adding certain stabilizing agents in small quantities. The stabilized forms have dominated the market. Polymerized sulfur or polysulfur is referred to as insoluble sulfur. There are no known solvents for insoluble sulfur; hence its name. Its main disadvantage is that it is hard to disperse well in unvulcanized rubber compound. It is also quite expensive compared to normal sulfur.
The degree of dispersion, in the unvulcanized rubber compound, of all of these compounding ingredients affects the properties of the vulcanized product. This is especially true of the vulcanizing agent. For the vast majority of products, such as tires, the best dispersion gives the-best product because of the homogeneity achieved.
Sulfur exists at room temperatures primarily as rhombic crystals. Other forms of sulfur, such as monoclinic crystalline sulfur, or polysulfur, are the normal primary forms which elemental sulfur assumes at certain higher temperature ranges. At room temperatures, these forms convert, or revert, to rhombic sulfur.
Polysulfur is called insoluble sulfur, especially in the rubber industry, and normal, non-polymeric or rhombic sulfur is called soluble sulfur, because it is soluble to a limited extent in most rubbers. The term "rubber" as used herein means any sulfur vulcanizable polymer. Sulfur vulcanizable polymers are primarily those polymers having carbon to carbon molecular chain structures, with some double bonds existing in their structure. These polymers are called unsaturated. The double bonds are the sites for sulfur vulcanization. The term "rubbery" as used herein means masses of matter that are not hard, brittle, or friable, but are plastic and/or elastic. The term "saturated rubbery polymers" means those rubbery polymers that do not contain sulfur vulcanizable bonds, such as ethylene-propylene rubber.
The amount of normal or rhombic sulfur that is soluble in rubber increases as the temperature increases. Typical rubber compounds contain from one to three parts of sulfur per one hundred parts of rubber hydrocarbon (rhc). The processing of unvulcanized rubber requires mechanical working of the rubber, which generates heat. The temperatures developed as a result of this processing are usually sufficient to dissolve the typical normal sulfur content. When the rubber cools to room temperature the solubility of the sulfur in rubber is exceeded, and a supersaturated solution ensues. This supersaturated portion of the sulfur tends to migrate to the surface of the rubber and crystallize. This condition is called "bloom" and is highly undesirable.
At room temperatures, surface blooming occurs primarily when the concentration of soluble sulfur in the rubber is between the limits of about 0.8 parts and 8.0 parts per 100 parts of rubber hydrocarbon. These limits vary among different compounds. Below the lower limit the sulfur is soluble. Above the upper limit the sulfur drops out of solution in the interior of most rubber compounds, forming micro-crystals throughout the mix. In some rubber compounds these micro-crystals grow to objectionable size, causing nonhomogeneity of properties throughout the vulcanized product.
Polysulfur or insoluble sulfur does not dissolve in rubber, and therefore does not bloom. However, the insoluble sulfur can revert to normal sulfur, and the rate of reversion is a time-temperature phenomenon which increases with temperature. Elemental insoluble sulfur can be stabilized by the addition of various substances, notably the halogens. This stabilized insoluble sulfur tends to remain polymeric at room and processing temperatures but it reverts to normal sulfur at the higher vulcanizing temperatures, thus becoming available for the vulcanization reaction.
Insoluble sulfur is normally supplied by the sulfur manufacturers in discrete particles, or powder. This powder is extremely fine, classically having a reported average particle size of 3 microns. These particles are considerably smaller than the particles usually supplied of normal sulfur. These smaller particles are desired because the dispersion of this form of insoluble sulfur is limited by the particle size supplied, unlike the dispersion of soluble sulfur. This very fine powder presents various processing difficulties. It tends to form dust clouds in the mixing room, which are both a health hazard and a safety hazard. Sulfur dust explosions are a known hazard in the rubber industry. A number of ways to reduce this dusting are mentioned in the literature. Also, the sulfur powder is difficult to disperse in rubber. The individual particles tend to agglomerate. Because of this, the powders are frequently mixed with a portion of a polymer or other matrix materials to form a masterbatch before being added to the final compound. These masterbatches usually contain fifty percent or more sulfur. This processing step adds to the cost. Since these discrete particles retain their identity during mixing, the best possible dispersion is limited by the size of the particles, unless their melting point is exceeded. However, when melted, the rate of reversion is very rapid and the reverted sulfur, of course, blooms, and the advantages of using insoluble sulfur are negated.
The prior art falls in three categories:
1. Insoluble Sulfur Powders PA1 U.S. Pat. No. 1,782,693 to Miller
U.S. Pat. No. 2,419,310 to Belchetz PA2 U.S. Pat. No. 2,419,309 to Belchetz PA2 U.S. Pat. No. 2,579,375 to Grove PA2 2. Sulfur Donors PA2 U.S. Pat. No. 4,621,118 to Schloman PA2 U.S. Pat. No. 2,989,513 to Hendry PA2 U.S. Pat. No. 2,481,140 to Morris
These patents deal with insoluble sulfur in a form that has distinct disadvantages. The present invention overcomes these disadvantages.
All of these patents teach a chemical reaction of sulfur with an organic compound to form sulfur donors. The crosslinking achieved using sulfur donors is distinctly different from that achieved using elemental sulfur. No long chain polymers of sulfur are contemplated or achieved. Therefore they are not pertinent.
3. Solutions of Normal Sulfur
This patent teaches solutions of normal sulfur in an organic resin. Long chain polymers of sulfur do not go into solution in any known substance. Hence it fails to teach or suggest anything concerning polymeric sulfur.
Recently an "improved product" has been introduced, that has a reported average particle size of 2 microns. The improvement is in the degree of dispersion afforded by the smaller particle size. However, as could be expected, the dusting problem has gotten worse with the "improved product", and the dispersion is, of course, still limited by the particle size. There is still a need for much better dispersion than is achieved by the "improved product", or that can be achieved by any product that must rely on the size of a particulate to maximize its dispersion.
It is generally known that rapidly cooling molten sulfur from a sufficiently high temperature produces a mass of polymeric sulfur that is in a rubbery or unhardened state. Sulfur in this rubbery state is in a metastable condition, and upon standing, it becomes hard and brittle. It can then be ground to form a powder. In some processes the molten sulfur is sprayed and thereby cooled, the individual droplets formed in the spraying are also initially rubbery, but are then permitted to become hard and brittle. Special handling is necessary to keep them as individual particles before hardening since the rubbery state tends to make them agglomerate. In other processes, sulfur vapor is forced under pressure into a liquid cooling medium. Again rubbery particles are first formed, and special handling is required until the particles harden. The rubbery unhardened state of insoluble sulfur has been considered undesirable because it has been a free flowing powder which has been desired.
I have discovered that the metastable, unhardened, rubbery or plastic state of insoluble sulfur can be preserved by lowering the temperature of the sulfur below the glass transition temperature of the rubbery mass. Upon returning to room temperature, the mass again becomes rubbery, but upon further standing it becomes hard and brittle. When in the rubbery state this sulfur can be dispersed in rubber to obtain the superior blends or dispersions of the present invention. If desired, the sulfur can be added to the rubber mix while it is still below its glass transition temperature. When exposed to the milling temperature it quickly warms up to the rubbery state.
Other elements in Group VI.sub.B of the Periodic Table of the Elements, notably Selenium and Tellurium, have many properties that are similar to sulfur. They have found limited use as vulcanizing agents for rubber, and they polymerize. It is well known that they polymerize themselves and that they form copolymers and terpolymers with sulfur. These polymers exhibit rubbery properties similar to those of the monopolymer of sulfur, and hence can be dispersed in similar fashions as the rubbery polymer of sulfur. All of these polymers are included in the term "polysulfur."
Other elements, such as Arsenic, can be included in the sulfur polymer chain, and a rubbery state can still be obtained. One skilled in the art could develop many such modifications of the sulfur polymer, and still obtain a rubbery polymer. These modifications are included in the substance of this invention. The terms "polymeric sulfur" and "polysulfur," as used herein, includes all of the modifications of the polymer that can be obtained in the rubbery state.
There is a need for ways to achieve superior dispersion of polymeric sulfur in rubber in a dust free manner. This need exists whether the sulfur is added to a rubber batch in the lower amounts needed for proper vulcanization, or in the higher amounts used in preparing masterbatches or intermediate batches, which are later added in the proportion needed to achieve the proper quantity of sulfur needed for vulcanization.